This invention relates to an upper bundle cleaning system for a nuclear power plant steam generator which eliminates the need to use chemical cleaning techniques.
Steam generators convert heat from the primary side of a nuclear power plant to steam on the secondary side so that the primary and secondary systems are kept separate. A typical generator is a vertical cylinder consisting of a large number of U-shaped tubes which extend from the floor or xe2x80x9ctube sheetxe2x80x9d of the generator. High temperature and pressure fluid from the reactor travels through the tubes giving up energy to a feed water blanket surrounding the tubes in the generator creating steam and ultimately power when later introduced to turbines.
Steam generators were designed to last upwards of forty years but in practice such reliability figures have proven not to be the case. The problem is that sludge from particulate impurities suspended in the feed water forms on the tubes which greatly affects the efficiency of the generator and can even cause the tubes to degrade to the point of causing fissures in the tubes. If radioactive primary fluid within the tubes seeps into the secondary side, the result can be disastrous. Plugging or otherwise servicing such fissures is time consuming and results in expensive down time during which power must be purchased from other sources at a great expense.
There are known methods for cleaning the tubes proximate the bottom of the steam generator using flexible lances and the like which clean the tubes using water under pressure, but since a typical steam generator can be thirty feet tall, it is difficult to reach the sludge at the upper levels of the tubes using water jets. So, chemical cleaning is used but there are several disadvantages. First, chemical cleaning is very expensive (from $5,000,000 to $10,000,000 per application) and requires an extended outage. Also, some corrosion of steam generator internals by the solvents used will occur during the cleaning. In addition, large quantities of hazardous, possibly radioactive waste may be generated. Disposal of this waste is very expensive. For these reasons, although many utilities have considered chemical cleaning, few plants have actually implemented chemical cleaning.
On the other hand, there are severe technical challenges faced when considering alternate cleaning methods. A typical steam generator has approximately 50,000 square feet of heat transfer area. The tube bundle is about 10 feet in diameter and 30 feet tall but the access alley in the middle of the tube bundle is only 3.5 inches wide and is interrupted by support plates approximately every 4 feet. There are flow slots through the support plates but they are very small in size, typically 2.75 by 15 inches. In addition, the access into the steam generator is limited to a six inch hand hole. Finally, inter tube gaps are only 0.406 wide or smaller.
Thus, the inherent design parameters of a typical steam generator make it difficult to incorporate water jet sludge lancing techniques at the upper tube bundles even though these techniques are adequate to clean the tubes at the level of the tube sheet at the bottom most portion of the steam generator. See, e.g. U.S. Pat. Nos. 4,700,662; 4,980,120; 4,887,555; 4,676,201; and 4,769,085.
It is therefore an object of this invention to provide an upper bundle steam generator cleaning system.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which cleans the generator from the top down thereby flushing deposits downward during the cleaning process.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which eliminates the need to use chemical cleaning techniques and overcomes the disadvantages inherent in chemical cleaning or which can be used in conjunction with chemical cleaning.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which adequately cleans the upper bundles of the steam generator using water under pressure even within the close confines of the tubes of the steam generator.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which successfully delivers sufficient water energy to remove scale and also distributes this energy in an efficient manner throughout the tube bundle.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which accomplishes cleaning remotely thereby overcoming the access restrictions of the steam generator as well as reducing exposure of personnel to radiation.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which maximizes cleaning effectiveness with a minimum use of water.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which minimizes the number of equipment moves during the cleaning procedure thereby reducing cleaning and hence outage time.
It is a further object of this invention to provide such an upper bundle steam generator cleaning system which utilizes both a bulk cleaning head and a rigid lance for intertube inspection and cleaning.
It is a further object of this invention to provide a method for cleaning the steam generator from the top down thereby flushing deposited downward during the cleaning process using the system of this invention.
This invention results from the realization that the most effective way to clean a steam generator is from the top down thereby flushing deposits downward as the cleaning process progresses; that there is an access path to the upper regions of the steam generator and that these upper regions of the steam generator can be cleaned without chemical cleaning techniques if a cleaning head which delivers fluid under pressure about the tubes can be deployed along this access path from the bottom of the steam generator and then raised up through successive flow slots into position; and furthermore that all the tubes on one side of the steam generator at one level can be cleaned from one flow slot thereby minimizing equipment moves and cleaning time; that a mechanical swinging arm can be used for bulk cleaning; that alignment of the nozzles of the swing arm first on one side of the tube gap and then on the other side to clean all the surfaces of the tubes maximizes cleaning effectiveness with a minimum use of water; and that a rigid lance which extends between the rows of tubes accomplishes effective inter-tube cleaning.
This invention features and may suitably comprise, include, consist essentially of and/or consist of an upper bundle steam generator cleaning system. There is a cleaning head means deployment and support device receivable through a hand hole of the steam generator which includes means to position a cleaning head about a flow slot of the upper bundles of the steam generator. The cleaning head means mounted with the support device directs fluid about the tubes of the upper bundles of the steam generator for cleaning the generator from the top down thereby flushing deposits downward during the cleaning process.
The cleaning head means deployment and support device includes a translation rail extending between a hand hole of the steam generator and the center tie rod along the blow down lane. There is a rotation stage including a number of vertically extendable cylinders pivotably mounted on the translation rail.
One head means includes means for directing fluid between the tubes from a flow slot such as an arm extendable along the direction of the flow slot wherein the arm includes a plurality of nozzles alienable with the spaces between the tubes. A number of the nozzles are opposed to each other for cleaning tubes on opposite sides of a flow slot at the same time and for balancing the thrust received by the arm. There are means for changing the pitch orientation of the nozzles and for rotating the arm to spray fluid about tubes proximate an adjacent flow slot.
Another cleaning head includes means for directing fluid between the tubes from in between the tubes. The means for directing fluid from between the tubes includes a lance extendable in between the tubes; the lance has a number of nozzles for spraying fluid about the tubes from in between the tubes. The lance is rotatable from a position about the support device for deployment of the cleaning head to a position between the tubes for cleaning.
The system further includes means for releasably supporting the cleaning heads about a flow slot. The means for releasably supporting includes rotatable fingers engagable with a side wall of a flow slot in a tube sheet support plate.
This invention also features an upper bundle nuclear power plant steam generator cleaning system for cleaning the generator from the top down thereby flushing deposits downward during the cleaning process, the system comprising: a cleaning head deployment and support device including means for positioning a cleaning head at a flow slot about the upper bundles of a steam generator; a bulk cleaning head affixable to the support device including means for directing fluid in between the tubes from the flow slot; and, a cleaning head lance also affixable to the support device including means for directing fluid in between the tubes from between the tubes.
The bulk cleaning head includes an arm extendable along the direction of a flow slot, the arm having plurality of nozzles alingable with the spaces between the tubes proximate the flow slot. A number of the nozzles are opposed to each other for cleaning tubes on opposite sides of the flow slot at the same time and balancing the thrust received by the arm. The system further includes means for changing the pitch orientation of the nozzle and for rotating the arm to spray fluid about tubes proximate ad adjacent flow slot. The cleaning head lance is extendable between the tubes and has a number of nozzles for spraying fluid about the tubes from between the tubes. The lance is rotatable from a position about the support device for deployment to a position between the tubes for cleaning. The system further includes means for supporting the bulk cleaning heads about a tube sheet support of the steam generator.
Finally, this invention features a method of cleaning a steam generator from the top down: a cleaning head device is inserted through a hand hold of the steam generator and deployed to a position about the upper tubes bundles of the steam generator; fluid is directed about the tube bundles proximate the cleaning head device; the cleaning head is successively lowered and repositioned proximate the next lower flow slot and fluid is again directed about the tube bundles proximate the cleaning head device thereby flushing deposits downward during the cleaning process.
The method includes spraying the tubes from the cleaning head located about one flow slot wherein the cleaning head is positioned to clean tubes proximate one flow slot; is moved to a position proximate an adjacent flow slot for cleaning tubes proximate that flow slot; and moved again to a position proximate another adjacent flow slot for cleaning tubes proximate that adjacent flow slot thereby achieving full coverage without retracting the cleaning head device through the flow slots.
The method also includes spraying water first on one side of the tube gap and then on the other side of the tube gap for effective cleaning and water conservation. Finally, the method includes filing the steam generator with water and lowering the level of water as the cleaning process progresses to provide an agitation action and enhanced cleaning as the water from the cleaning head spray nozzles strikes the surface of the water within the steam generator.